Conical sputtering target

ABSTRACT

A hollow cathode magnetron for sputtering target material from the inner surface of a target onto an off-spaced substrate. The magnetron is in the shape of a truncated cone, also known as a conical frustum. The target cone is backed by a conical cathode maintained at a predetermined voltage for attracting gas ions into the inner surface of the target cone to sputter material therefrom. The inner surface of the cone is bounded at its inner and outer edges by magnetic pole pieces orthogonal to and extending inwardly and outwardly of the cone surface. The magnetic path is completed by a conical magnet surrounding the target and conical electrode and magnetically connected to the pole pieces Lo form a magnetic cage. Lines of magnetic flux extending above the target surface between the pole pieces are substantially parallel the target surface, providing uniform erosion over the entire surface. Preferably, the conical magnet is tapered so that some lines of magnetic flux terminate in the target surface, maintaining thereby a uniform flux density and consequent uniform erosional intensity over all portions of the surface of the target. Sputter coatings on planar targets can achieve areal thickness nonuniformities of less than ±0.2%.

The present invention relates to apparatus for low-pressure depositionof materials, and more particularly to apparatus for sputter coating,and most particularly to a conical target and associated magnetronapparatus for making sputtered coatings having extremely high thicknessuniformity.

Thickness uniformity requirements for sputtered coatings are becomingincreasingly stringent. For example, the so-called LO layer in digitalvideo disks must have a reflectivity variation of less than ±5% over thedisk surface, requiring an equivalent uniformity in sputtered layerthickness. In some applications, only a minimum coated thickness orreflectivity is required, which may be readily achieved by coating toexcess. However, this can shorten the useful lifetime of the sputteringtarget, and in the case of precious metals such as gold or silver suchwasteful coating can be very expensive.

It is known in the art to move or rotate a substrate during coating toreduce areal variation in coated thickness, but newer generations ofapparatus, such as cluster tools and optical disk coaters, typicallycoat a single substrate at a time without rotation. In someapplications, the coating exposure time is less than one second. Thismeans that the traditional method of moving the substrate relative tothe source to produce more uniform coatings is either not possible ornot practical.

One way of achieving good uniformity when source and substrate are fixedwith respect to each other is to use diode sputtering. This processremoves material uniformly from a planar target and deposits itefficiently on a closely-spaced parallel substrate. FIG. 1 is a graph ofcalculations based on a uniform cosine distribution of material fromeach elemental area of a planar target 21 cm in diameter. This diameterwas chosen to illustrate the possibility of good uniformity on asubstrate 12 cm in diameter, such as a digital video disk. FIG. 1 showsthat a thickness uniformity of approximately ±1% can be achieved throughdiode sputtering.

Even though diode sputtering can result in good uniformity on circularsubstrates, it is almost never used because of several seriousdrawbacks. First, it produces relatively low sputtering rates atreasonable power densities, which translates into long coating times andlow throughput. Second, diode sputtering requires high sputteringpressures, which results in less desirable film properties. Third, thediode sputtering process generates electrons which are accelerated awayfrom the target at high energy which can damage or excessively heat thesubstrate being coated.

Magnetron sputtering overcomes these limitations such that virtually allmodern sputtering is done with magnetron cathodes. These devices usemagnetic fields to confine electrons to the vicinity of the targetsurface, resulting in more efficient use of electrons and higher plasmadensities. This translates into lower operating pressures, less electronbombardment of the substrate, and higher deposition rates.

In a magnetron cathode, the erosion rate is highest where the magneticfield is parallel to the target surface. Therefore, in order to use asmuch of the target as possible and to maximize the uniformity ofdeposition as well, one useful design confines the electrons with acombination of a parallel magnetic field and electric field, known inthe art as a hollow cathode configuration. An example of one such designis disclosed in U.S. Pat. No. 4,486,287.

A disadvantage of ail planar magnetrons, in which the target isessentially a flat surface, is that the magnetic field lines must passthrough the plane of the target in the inner portion of the target. Thismakes it impossible to magnetron sputter the surface of a planar targetuniformly over its entire area, since there is always a central portionfrom which no sputtering takes place (whether or not target material islocated there). The consequence of this on film uniformity issignificant and is highly undesirable. FIG. 2 shows the effect ofeliminating a central portion 5 cm in diameter of the previouslydescribed 21 cm target, such as is the case in a typical planarmagnetron. Even though this central portion represents a relativelysmall percentage of the target area, the optimum uniformity on a 12 cmsubstrate is significantly reduced as a result of not using the centralportion of the target. Moving farther away can improve uniformity but ata severe cost in materials utilization and deposition rate.

Minimizing the extent of this central portion is the subject of U.S.Pat. No. 5,597,459. In this configuration, the size of the magnetic polepiece is minimized to reduce the non-sputtered area. However, as shownin FIG. 3, even if the non-sputtering central area is reduced to adiameter of only 3 cm, there is still a significant penalty in terms ofuniformity loss.

Achieving high uniformity over stationary substrates, therefore,presents o substantial problem for planar magnetrons. Several patents(see, for example, U.S. Pat. Nos. 4,595,482; 4,606,806; and 4,840,347)disclose the simultaneous use of two independent concentric targets toachieve better uniformity than a single target alone can produce.However, such designs are relatively complex and sometimes requireindependent control of power to each target. Moreover, they do not usethe hollow cathode concept, which means that the targets will not erodeuniformly over their surfaces, resulting in relatively poor targetutilization, as well as a distribution of sputtered material whichchanges 0the areal uniformity over the lifetime of the target.

There is a class of magnetron sputtering cathodes which provide theadvantages of essentially uniform material erosion over virtually theentire target surface and a stable coating profile over the entiretarget lifetime. These devices, known in the art as inverted cylindricalmagnetrons, also use the hollow cathode confinement technique (see, forexample, U.S. Pat. Nos. 3,884,793; 3,995,187; 4,030,996; 4,031,424;4,041,353; 4,111,782; 4,116,793; 4,116,794; 4,132,612; and 4,132,613).However, instead of being a planar surface, the target typically is theinside surface of a cylinder. Such cathodes are available commerciallyfor coating the outsides of wires, fibers, and three dimensional objectswhich are placed in or passed through the cylinder.

We have found, completely unexpectedly, that a flat, circular substrateplaced with its surfaces normal to the axis of such a cylindricalmagnetron and beyond the end of the cylinder can receive a relativelyuniform coating (see the proceedings of the 39th Annual TechnicalConference of the Society of Vacuum Coaters, 1996, p.97). For example,in FIG. 4 is shown a calculation of the coating profile produced by acylindrical magnetron 10 cm long and 21 cm in diameter sputtering onto aflat substrate surface placed at three different distances from the endof the cylindrical target. Surprisingly, at a distance of 2.5 cm fromthe end, the uniformity variation is slightly better than ±1%. However,a disadvantage of using cylindrical magnetron sputtering to coat a flatsubstrate is that, unless substrates are placed at both ends,substantially half of the sputtered material is lost out the unused endof the cylinder.

It is a principal object of the invention to provide an improvedapparatus for making highly uniform sputtered coatings on planarsubstrates.

It is a further object of the invention to provide apparatus for makingsputtered coatings on planar substrates with reduced waste of targetmaterial.

It is a still further object of the invention to provide an improvedhollow cathode magnetron which sputters uniformly over its entire targetsurface.

Briefly described, a hollow cathode magnetron in accordance with theInvention is provided in the shape of a truncated cone, also known as aconical frustum. The target cone is backed over its outer surface by aconical cathode maintained at a predetermined voltage for attracting gasions into the inner surface of the target cone to sputter materialtherefrom. The inner surface of the cone is bounded at its inner andouter edges by magnetic pole pieces orthogonal to and extending inwardlyand outwardly of the cone surface. The magnetic path is completed by oneor more discrete magnets or a conical magnet surrounding the target andmagnetically connected between the pole pieces. Lines of magnetic fluxthus extend between the outer and inner pole pieces above andsubstantially parallel with the target surface, providing therebyerosion over the entire surface. Preferably, the conical magnet istapered along its length so that some lines of magnetic flux terminatein the target surface, maintaining thereby a substantially uniform fluxdensity over the surface and consequent uniform erosional intensity overall portions of the target. Sputter coatings on planar targets canachieve areal thickness nonuniformities of less than ±0.2%.

The foregoing and other objects, features, and advantages of theinvention, as well as presently preferred embodiments thereof, willbecome more apparent from a reading of the following description inconnection with the accompanying drawings in which:

FIG. 1 is a calculated graph of sputtered deposition on a planarsubstrate from a prior art planar diode sputtering target having nocenter opening, showing relative thickness of deposition at variousradial positions on the substrate, conducted at three differenttarget-substrate spacings;

FIG. 2 is a calculated graph like that shown in FIG. 1, except that theprior art planar target is a magnetron and is provided with a centeropening 5 cm in diameter;

FIG. 3 is a calculated graph like that shown in FIG. 2, except that theprior art planar magnetron target is provided with a center opening 3 cmin diameter;

FIG. 4 s a calculated graph of sputtered deposition on a planarsubstrate orthogonal to the axis of a prior art cylindrical magnetrontarget (zero degree inclination angle of the substrate to the sputteringsurface), conducted at three different spacings of the substrate fromthe end of the cylinder;

FIG. 5 is a calculated graph of a sputtered deposition on a planarsubstrate orthogonal to the axis of a frusto-conical magnetron target inaccordance with the invention having a 90 degree included cone angle (45degree inclination angle of the substrate to the sputtering surface),conducted at three different spacings from the large end of the cone;

FIG. 6 is a cross-sectional view of a frusto-conical magnetron inaccordance with the invention;

FIG. 7 is a calculated graph showing the effect of changing the frustumlength of he conical target in FIG. 6;

FIG. 8 is a calculated graph showing the effect of changing the includedangle of the frusto-conical target in FIG. 6;

FIG. 9 is a cross-sectional view of a frusto-conical magnetron having aspot target suitable for RF-powered sputtering; and

FIG. 10 is a cross-sectional view of a frusto-conical magnetron combinedwith an ion beam deposition source.

Referring to FIG. 1, a prior art circular planar diode sputtering targetmay be spaced apart from and parallel with a circular substrate 12 cm indiameter to be sputter coated. Such an arrangement is well known in theart and therefore the apparatus is not illustrated here. As shown inFIG. 1, at the closest practical spacing 1.5 cm, depositionnonuniformity is about 2% (±1) from center to edge (0 to 6 cm) of thesubstrate.

Referring to FIGS. 2-3, a prior art circular planar magnetron targethaving a center opening provides uniformity substantially interior tothat from the planar diode as shown in FIG. 1. Even at the farthestspacing examined, 4.5 cm, center-to-edge nonuniformity of ±8% isobtained from a planar magnetron having a central aperture 5 cm indiameter. When the central aperture is reduced to 3 cm, nonuniformity isdiminished but is still about ±2.5% at the farthest spacing. FIG. 3 alsoshows that further uniformity improvement using a planar magnetron isnot achievable through spacing changes: although coated uniformity ofthe inner portion of the substrate is improved with increased spacingbetween the target and substrate, coated thickness toward the outer edgeof the substrate falls off rapidly with increasing radius.

Referring to FIG. 4, a hollow cylindrical magnetron, sputtering from itsinner surface onto a substrate orthogonal to the cylindrical axis andbeyond an end of the cylinder, can provide a coating havingcenter-to-edge nonuniformity of about ±1.5%. The coated profile is verysensitive to spacing of the substrate from the end of the cylinder andexhibits cross-over falloff similar to that seen with a planar magnetronin FIG. 3. Because the surface of the cylindrical target is orthogonalto the plane of the substrate, the surface is said to have "zeroinclination angle" to the substrate, an important concept in a noveltarget discussed hereinbelow.

As noted above, a disadvantage of using a cylindrical magnetron tosputter coat a flat substrate is that, unless substrates are placed atboth ends, sputtered material is lost out the unused end. We reasonedthat this loss could be minimized or even eliminated by narrowing theunused end of the cylinder, or in other words by forming the magnetrontarget as a truncated cone rather than a cylinder. Surprisingly, wefound that the potential for coated uniformity is significantly betterthan that for a cylindrical target having similar dimensions. As shownin FIG. 5, by making the target surface a frustum cone with an includedangle of 90 degrees, the coated uniformity of a substrate placedOrthogonal to the cone axis and beyond the larger, or base end, of thecone, is even greater than can be achieved with the original planardiode target as shown in FIG. 1, the resulting non-uniformity capable ofbeing substantially less than ±1%. Also, unlike a planar magnetron, afrusto-conical shape makes possible the closing of magnetic flux linesthrough a relatively Mange diameter at the narrow end of the cone.

Referring to FIG. 6, a frusto-conical sputtering magnetron 10 inaccordance with the invention has a conical target 12 formed of anymaternal suitable for being sputter deposited on facing surface 14 of asubstrate 16, for example but not limited to, metals such as aluminum,gold, and silver. Typically, substrate 16 may be a cask having a radiusR, the disk being disposed at a spacing 36 from target 12 and beingcoaxial with and orthogonal to axis 17 of target 12. Backing andsupporting target 12 is a cooling racket 18 having a coolant passageway20 for circulation of a coolant liquid such as water. Jacket 18 alsoserves as an electrode, typically a cathode, for generation of anelectric field and plasma for sputtering in a fashion well known tothose skilled in the art and therefore not illustrated herein.Preferably, no bonding is provided between target 12 and jacket 18,since the target expands and thereby clamps tightly to the backingjacket due to temperature rise in the target which occurs duringsputtering upper and lower wings 22,24 are reverse cones which arepreferably substantially orthogonal to the inner surface of target 12which capture target 12 and jacket 18 therebetween. Wings 22,24 extendboth inward and outward of target 12 and are physically connectedoutboard of jacket 18 by one or more magnets 26 to form a magnetic cage27 similar to that disclosed in U.S. Pat. No. 3,919,678, herebyincorporated by reference, for use in cylindrical magnetrons. Magnet 26may be a continuous conical magnet or a conical cage formed of aplurality of individual magnets. Preferably, magnet 26 is a permanentmagnet, although electromagnets are within the scope of the invention.

Plasma confinement car the sputtering surface 13 of target 12 isachieved by a combination of wings 22,24 which are maintained at thetarget potential and a magnetic field 28 whose component parallel to thetarget surface is essentially uniform. Outer anode 30 and inner anode30' are electrically isolated from cathode 18 and target 12 byinsulation 32 which also acts as a vacuum seal between the interior ofthe magnetron during sputtering and its exterior. Inner anode 30 may beformed with or without an axial opening 31. It is a characteristic ofmagnetron 10 that all lines of magnetic flux lie in planes which includethe axis of the target cone, and therefore no lines of flux cross fromone plane to another (have no azimuthal component). The flux lines thusconverge toward the narrow end of target 12. In order to erode all areasof the target surface at a uniform rate, it is necessary to maintain themagnetic field at uniform field strength parallel to the target surface.This can be achieved by using a conical magnet 26 which tapers instrength from its outer end to inner end, either through physicaltapering of magnet thickness as shown in FIG. 6 or through a magnetwhose magnetization per unit volume is varied along its length. Thisarrangement causes some lines of magnetic flux 34 to enter surface 13without reaching lower wing 24, thus reducing the magnetic flux densityover the shorter-radius portions of the target surface. This isnecessary because the lines of flux converge toward the apex of thecone, and otherwise the flux density would progressively increase alongthe length of the frustum. Another means for attenuating the magneticintensity along the target surface is to provide magnet 26 as a conicalmagnet which is magnetized parallel to the surface of the cone. Othermethods may be obvious for producing a magnetic field that isessentially uniform in strength parallel to the target surface and hasno azimuthal component, and such methods are within the scope of theinvention.

Effective use of a conical magnetron in accordance with the inventionrequires the mutual optimization of included cone angle, target length,and substrate spacing from the target, along with a combination ofmagnetic and electrostatic electron confinement that results ineffectively uniform target erosion. Referring to FIG. 7, the result ofchanging frustum length at an included cone angle of 90 degrees(inclination angle a of 45 degrees) is shown. Under these conditions, aspacing of about 9.9 cm is optimum and can provide a coated thicknessuniformity variation of about ±0.7%. In general, the most effectivemaximum target radius, that is, the radius of the base of the cone, isbetween about 1.5 and 2.0 times the radius R of the substrate. The mosteffective length of the frustum of the cone between the ends thereof isbetween about 0.5R. and about 1.5R. Combining these two parameters, itis found that the highest depositional uniformity is produced when thefrustum length (shortest distance between the large and small openingalong the surface of the cone) is about one half the diameter of thelarge opening. Since uniformity is also a function of substrate spacing,optimization of uniformity is readily achieved empirically by firstproviding a target having these proportions and then varying spacing.

Referring to FIG. 8, the system is surprisingly insensitive to largevariations in included cone angle. For a substrate 6 cm in radius, suchas a digital video disk, a 90 degree cone angle is probably optimum(same data as shown in FIG. 7) , although for smaller substrates such asan 8 cm disk (4 cm radius) a 60 degree included cone angle (60 degreeinclination angle) can provide a coated thickness uniformity variationof less than about ±0.2%. In general, a usable includes cone angleshould be between about 60 degrees and about 120 degrees.

A conical magnetron 38 in accordance with the invention can beconfigured For RF sputtering, as shown in FIG. 9. Conical target 12 isprovided an inner conical cathode 40 and an outer conical cathode 42,separated by additional wings 44,44' which are themselves separated byan electrical insulator 46. The two cathodes are connected across aconventional RF power source 48.

Another advantage of a frusto-conical magnetron is that the opening atthe narrow end can permit use of other equipment simultaneous during asshown in FIG. 10. For example, a Kaufman type ion source 50 may bemounted in such a way that it can bombard the growing film on thesubstrate surface with ions 52 of controlled type, energy, and dose.This can be very useful film properties. The opening also permits thedeposition of insulating compounds formed by rapidly sputtering a metaltargets and simultaneously bombarding the growing film with a reactivegas. In some applications it may be necessary to alter the included coneangle to accommodate the additional apparatus, Or example, in FIG. 10the included cone angle is 60 degrees, which is within the range of coneangles capable of providing excellent depositional uniformity.

A further advantage of a frusto-conical magnetron is that the componentsmay readily be held together by atmospheric pressure. With suitablebacking (not shown) of the magnetic cage, a chamber seal at point 37 inFIG. 6 can lock the components together while the interior of themagnetron is under vacuum and also can permit rapid removal andreplacement of a used target when the vacuum is broken simply by firstremoval of anode 30 and wing 22. In known magnetrons, target replacementcan be a time-consuming, and therefore costly, operation.

From the foregoing description it will be apparent that there has beenprovided an improved magnetron for sputter coating of planar substrates,wherein a substantially uniform magnetic field is maintained over ailportions of the surface of a frusto-conical target. Variations andmodifications of the herein described magnetron, in accordance with theinvention, will undoubtedly suggest themselves to those skilled in thisart. Accordingly, the foregoing description should be taken asillustrative and not in a limiting sense.

What is claimed is:
 1. A magnetron for sputtering a target material ontoa substrate having a radius R to form a coating of the target materialupon a surface of the substrate, comprising:a) a frusto-conical targetoff-spaced from said substrate and having an inner surface for beingsputtered, said conical target being a conical frustum having an axiscentrally disposed through first and second parallel planes truncatingsaid conical target to define respectively larger and smaller openingsin said target, said target having a frustum length determinable alongsaid inner surface between said openings and having an included coneangle defined by the angle between the locus of opposite sides of saidinner surface included in a plane inclusive of said axis; b) a firstelectrode disposed adjacent said conical target opposite said innersurface; c) a magnetic cage receivable of said frusto-conical target andsaid first electrode for providing a magnetic field between said innersurface and said substrate surface, said magnetic field containing fieldlines of magnetic flux, said magnetic cage including first and secondpole pieces disposed substantially orthogonal to and extending inwardlyof said inner surface, said magnetic cage including a conical magnetwhich tapers in magnetic strength between its outer and inner radii; andd) a second electrode disposed adjacent said magnetic cage forcooperating with said first electrode and said magnetic cage to providean electric plasma over said inner surface.
 2. A magnetron in accordancewith claim 1 wherein each line of flux in said magnetic field iscontained within a plane containing said axis.
 3. A magnetron inaccordance with claim 1 wherein a component of said magnetic fieldparallel to said inner conical surface is substantially the samestrength at all points on said inner surface.
 4. A magnetron inaccordance with claim 1 wherein said inner surface of saidfrusto-conical target has a maximum radius of between about 1.5 R andabout 2.0 R.
 5. A magnetron in accordance with claim 1 wherein thelength of the sputtering area along the frustum said frusto-conicaltarget is between about 0.5 R and about 1.5 R.
 6. A magnetron inaccordance with claim 1 wherein said substrate is disposed orthogonallyto said axis said frustro-conical target at a distance of between about0.1 R and about 0.4 R from the base of said frustro-conical target.
 7. Amagnetron in accordance with claim 1 wherein the included cone angle ofsaid frusto-conical target is between about 60 degrees and about 120degrees.
 8. A magnetron in accordance with claim 1 wherein saidfrustro-conical target and said first electrode are retainable withinsaid magnetic cage by atmospheric pressure.
 9. A magnetron in accordancewith claim 1 further comprising an ion beam source disposed forprojection of said ion beam through said smaller opening in said target.